1. Field of the Invention
The present invention relates to a method and apparatus for controlling flash emission, and more particularly, to a method and apparatus for controlling flash emission in conjunction with an image sensor using a rolling shutter, such as a complementary metal-oxide semiconductor (CMOS) sensor.
2. Description of the Related Art
Currently, digital cameras mostly include a flash lamp using a xenon (Xe) tube. A flash lamp is used to correct for the intensity of light illuminating a subject when an image sensor is exposed in a dark environment. In general, flash emissions are corrected by controlling an emission time of flash light. In more detail, a subject may be photographed too brightly if the emission time is too long and too dimly if the emission time is too short. Since the brightness of a subject is controlled based on an emission time of flash light, an algorithm for controlling the emission time is required and two flash emission time control algorithms mainly used are as described below.
First, in an illumination algorithm using an illumination sensor, when an image is captured, flash light is emitted and the brightness of a subject is measured by using the illumination sensor. If the brightness of the subject reaches a preset brightness level due to the flash emission, the flash emission is stopped. The subject is photographed for the emission time of flash light.
Second, in a pre-emission algorithm, flash light is previously emitted at a weak level so as to measure the brightness of a subject, an emission time of flash light for making the subject reach a preset brightness level is calculated, and the subject is photographed by emitting flash light for the calculated emission time.
Meanwhile, a complementary metal-oxide semiconductor (CMOS) image sensor using a rolling shutter may be operated in a live-view mode. Here, the rolling shutter sequentially exposes pixels in one frame. In more detail, the pixels are exposed at different times. When the CMOS image sensor using a rolling shutter is operated, if a subject or a user moves while photographing a subject, image distortion may occur due to different exposure times, which is referred to as a rolling shutter effect.
If the above-described algorithms are applied to a CMOS image sensor, problems may occur as described below. Although the illumination algorithm may be performed in the same manner as a conventional algorithm, if an illumination sensor is used, a cost for the illumination sensor is added, partial image saturation may not be easily corrected, and the illumination sensor has to be additionally adjusted in a manufacturing process of a photographing apparatus.
Although the pre-emission algorithm is generally used, partial image exposure is caused in a CMOS image sensor during pre-emission due to a rolling shutter. In more detail, if the brightness of an image is partially measured during the pre-emission, the brightness may not be normally calculated during the pre-emission, which is a common problem of operating image sensors using a rolling shutter.